1. Field of the Invention
The present invention relates to a cornea imaging apparatus for imaging corneal endothelial cells by means of directing illumination light into an eye under examination and receiving light reflected from the cornea thereof, and a cornea imaging method.
2. Description of the Related Art
Conventionally, observation of the cornea of the eye, and in particular of the cellular status of the corneal endothelium, has been commonly carried out in determining a presence of ocular disorders or making a diagnosis of postoperative prognosis.
In order to observe such cellular status of the corneal endothelium, there has been known a cornea imaging apparatus capable of imaging corneal endothelial cells without contacting the eye under examination. This cornea imaging apparatus is designed to direct a slit beam of illumination light (slit light flux) from an illumination optical system into the cornea of the eye under examination at an angle and receive the light reflected from the cornea by an imaging optical system to take images of the corneal endothelial cells.
Meanwhile, in taking images of the corneal endothelium, there are sometimes needs for covering a wide area including not only the center portion of the cornea but also the peripheral portion thereof. To meet such requirements, Japanese Patent No. 2580464 has proposed a structure, wherein images of the corneal periphery are taken by irradiating a slit light flux for taking images on the corneal periphery by letting the test subject fixate in an oblique direction away from straight ahead and imaging the reflected light flux with an imaging optical system.
However, in the cornea imaging apparatus described in Japanese Patent No. 2580464, clear images of corneal endothelium could not be obtained in some cases since imaging of the corneal endothelium in the corneal periphery is performed through an XY alignment of the imaging optical system using specularly reflected light from the corneal epithelium (also called ectocornea) in the same way as for the center portion of the cornea. As anatomically evident, this is caused by the difference in curvature of the corneal endothelium from that of the corneal epithelium, and in the peripheral portion, performing the XY alignment using the specularly reflected light from the corneal epithelium in the same direction can move the alignment away from the optimum position of the corneal endothelium, although the XY alignment of the corneal endothelium can be performed using the specularly reflected light from the corneal epithelium at the corneal center.
In other words, under a state where the test subject fixates straight ahead to get images of the corneal center taken, the optical axis (marked with ●) of the XY alignment light reflected from the corneal epithelium is aligned with the optical axis (marked with x) of the light reflected from the corneal endothelium, as shown in FIG. 18A, so that the corneal endothelium and corneal epithelium share the same optical axis by the performance of an XY alignment within the imaging area marked with □, thus enabling more accurate imaging of the targeted endothelium. However, under a state where the test subject fixates in an oblique direction to get images of the corneal periphery taken, the optical axis (marked with x) of the light reflected from the endothelium can be moved from the optical axis (marked with ●) of the XY alignment light reflected from the corneal epithelium so that the accuracy of endothelium imaging is significantly degraded even with the XY alignment due to the difference of optical axes between the corneal endothelium and corneal epithelium.
In summary, in order to take clear images of the corneal endothelium, it is necessary to do it after moving the setting of the imaging area (marked with □) by a given distance from the center point (marked with ●) of the XY alignment in the shift direction of the corneal endothelium (marked with x) in FIG. 18B.
For that reason, in case of taking wide-range cornea images in a conventional way with a cornea imaging apparatus and failing to obtain clear images, the imaging process was repeated until satisfactory images are obtained by having the tester move the relative position of the imaging optical system against the eye under examination in the XY direction for positioning. That imposed a significant burden on the tester and the test subject and much time was required for an inexperienced tester to complete the imaging, which is likely to inflict pain on the test subject.
To deal with these problems, U.S. Pat. No. 5,557,351 and Japanese Patent No. 3338529 propose a cornea imaging apparatus that presets the amount of correction of the XY alignment against the eye under examination corresponding to the position in the oblique direction that the eye under examination fixates so as to adjust the XY alignment position determined by the XY alignment signal obtained from the light reflected from the corneal epithelium by the amount of correction corresponding to the selected fixation target. Here, it is conceived that the preset amount of correction of the XY alignment can be specified from anatomical data based on, for example, the theory of difference in curvature between the corneal endothelium and corneal epithelium in the shift direction α in FIG. 18B.
However, investigations performed by the inventor revealed that there were many cases where wide range cornea images cannot be taken with enough accuracy even if the XY alignment following the technology described in U.S. Pat. No. 5,557,351 and Japanese Patent No. 3338529 of the prior art is adopted. Upon examination of any possible cause by the inventor, it was revealed that individual differences exist in the curvature radius of the corneal endothelium and corneal epithelium, and the extent of these differences is sometimes significant enough to affect the imaging accuracy. What is especially noteworthy is the fact that many of those who need imaging of their corneal endothelium are patients who have ocular disorders or require postoperative follow-up, not the general public, and that the curvatures of the corneal endothelium and conical epithelium often differ much in those patients.
Considering these facts, in the cornea imaging apparatus described in the above U.S. Pat. No. 5,557,351 and Japanese Patent No. 3338529 that adopt the preset amount of correction based on the assumption that the individual differences in the curvature radius of the corneal endothelium are surely small enough not to affect the imaging accuracy in light of the anatomical statistical data, it occurs relatively often that the amount of correction is inappropriate, and after all, the tester needs to fine-tune the imaging position in actual on-site uses, and in that sense, the cornea imaging apparatus described in U.S. Pat. No. 5,557,351 and Japanese Patent No. 3338529 as the prior art should be considered nothing more than an imaging apparatus equipped with a semi-automatic alignment mechanism.